As a future communication standard of LTE (Long Term Evolution), 3GPP (3rd Generation Partnership Project) is presently developing standardization of LTE-Advanced. In a LTE-Advanced system, a carrier aggregation (CA) technique is introduced to achieve a higher throughput than a LTE system while ensuring backward compatibility with the LTE system. In the carrier aggregation, a component carrier (CC) having the maximum bandwidth of 20 MHz supported by the LTE system is utilized as a basic component, and it is designed to achieve communications in a wider band by employing such multiple component carriers simultaneously.
In the carrier aggregation, a user equipment (UE) can communicate with abase station (evolved NodeB: eNB) by employing the multiple component carriers simultaneously. In the carrier aggregation, a primary cell (PCell) having high reliability for securing connectivity to a user equipment and a secondary cell (SCell) additionally configured to the user equipment being in connection to the primary cell are configured.
The primary cell is like a serving cell in the LTE system and is a cell to secure connectivity between the user equipment and a network. Specifically, in the primary cell, the user equipment can receive a PDCCH (Physical Downlink Control Channel) and a PDSCH (Physical Downlink Shared Channel) and transmit a PRACH (Physical Random Access Channel), a PUCCH (Physical Uplink Control Channel), a PUSCH (Physical Uplink Shared Channel) and a SRS (Sounding Reference Symbol). Also, if the user equipment changes the primary cell, the user equipment must perform handover. On the other hand, the secondary cell is a cell configured to the user equipment in addition to the primary cell. Addition and deletion of the secondary cell is performed by a configuration in RRC (Radio Resource Control).
See 3GPP TS 36.213 V.11.0.0 (2012-09) for the carrier aggregation, for example.